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And in that screen you can see a list of all the valid users, it should be filled with around a dozen dummy accounts and your account. Copy paste your or any of the "User Principal Name" in that list to your request link and this time the request will be valid.
Getting userPrincipalName and ID via GET Request (Optional)
Optionally, you can extract both the "User Principal Name" and their ID, by doing a GET request to the link,
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